Conventionally, electricity storage devices capable of being charged/discharged are used as: a power supply for hybrid cars powered by using two types of energy being gasoline and electricity, and for various electronic equipments, particularly for mobile communications equipments, portable electronic equipments, and the like; an uninterruptible power supply; and the like. With the recent dissemination of hybrid cars and electronic devices, demand for enabling higher performance in electricity storage devices is becoming extremely high. In response to this demand, research and development related to electric double layer capacitors, which is one type of electricity storage device, are being actively pursued. Electric double layer capacitors have features of high output and excellent repeated charge/discharge life characteristics, and application mainly for use due to high output is being highly anticipated. However, electric double layer capacitors have a lower capacity, that is, a lower energy density, when compared to secondary batteries.
For enabling higher capacity in electric double layer capacitors, for example, a positive electrode active material of an electric double layer capacitor and a negative electrode active material of a lithium-ion battery are selected, and then, various studies are made regarding: optimization performed separately for a positive electrode active material and a negative electrode active material; optimization performed for the positive and negative electrode active materials as a combination; and the like. For the negative electrode active material, for example, carbon materials such as graphite, polyacene, and the like in a crystalline or an amorphous state are being considered. These carbon materials are materials that can reversibly absorb and desorb (hereinafter referred to as absorb/desorb) lithium ions by charging/discharging. A large number of proposals are being made regarding: a method for producing these carbon materials; an electricity storage device having a positive electrode active material of an electric double layer capacitor such as an activated carbon and a negative electrode containing these carbon materials, combined together; and the like (for example, refer to Patent Documents 1 and 2).
In addition, attempts are being made to enable high capacity in electricity storage devices by using a negative electrode active material that has a higher capacity density than a carbon material. For this kind of negative electrode active material: a negative electrode active material resulting from reforming a carbon material; a negative electrode active material other than a carbon material; and the like can be given. For the negative electrode active material resulting from reforming a carbon material: “negative electrode active materials in which optically-anisotropic carbonaceous material other than graphite is subjected to activation; and negative electrode active materials whose surfaces have been partially or entirely covered with a carbon material and/or a silicon material” are proposed (for example, refer to Patent Document 2). For the negative electrode active material other than a carbon material, metal oxides such as a tin oxide, a silicon oxide, and the like are proposed (for example, refer to Patent Documents 3 and 4).
Patent Documents 2 to 4 disclose the combination of: a negative electrode active material disclosed in each of the patent documents; and a positive electrode active material which is an activated carbon. In particular, Patent Documents 3 and 4 disclose that an electricity storage device having excellent over-discharge characteristics can be obtained by combining: a negative electrode active material that is a non-carbon material such as a silicon oxide and a tin oxide; and a positive electrode active material that is an activated carbon. More specifically, in Example 1 of Patent Document 3, a negative electrode material mixture is prepared by mixing: silicon monoxide particles (a negative electrode active material) that has undergone grounding and granulation to have a particle size of 44 μm or less; graphite (a conductive material); and polyacrylic acid (a binder), in a weight ratio of 45:40:15, respectively. This negative electrode material mixture is pressure-molded, and thus a negative electrode pellet having a diameter of 4 mm and a thickness of 0.19 mm is fabricated. This negative electrode pellet is fixed to a negative electrode case by a conductive resin binder that also serves as a negative electrode current collector. An electricity storage device including this negative electrode pellet is capable of operating in a voltage region that includes an over-discharge region ranging from 2 V to an end-of-discharge voltage of 0 V. However, the charge/discharge rate is extremely low with a 200-hour rate (rate of 0.005 C), and thus, output characteristics are poor.
Meanwhile, lithium secondary batteries which are largely used in electronic equipments such as portable electronic devices, generally have a charge/discharge rate of about a 10-hour rate to a 0.5-hour rate (rate of 0.1 C to 2 C). Therefore, electricity storage devices of Patent Documents 3 and 4 apparently cannot be used as alternatives for a lithium secondary battery. In addition, electric double layer capacitors capable of being instantly charged/discharged at a large current generally have a charge/discharge rate of about a 0.002-hour rate to a 0.02-hour rate (rate of 500 C to 50 C). Therefore, electricity storage devices of Patent Documents 3 and 4 cannot be used as alternatives for an electric double layer capacitor as well. In this manner, electricity storage devices disclosed in Patent Documents 3 and 4 are high in voltage and capacity, but are limited in use due to a low charge/discharge rate and poor output characteristics.
In contrast, for the positive electrode active material, being considered is an organic compound that is capable of being oxidized and reduced, and has higher capacity compared to the activated carbon which is currently widely used. For the organic compound capable of being oxidized and reduced, being proposed are an organic compound having a π conjugated-electron cloud (for example, refer to Patent Documents 5 and 6), an organic compound having a radical (for example, refer to Patent Document 7), and the like. However, these Patent Documents do not report anything regarding a combination of: a positive electrode active material that is an organic compound capable of being oxidized and reduced; and a negative electrode active material that is a non-carbon material.    Patent Document 1: International Publication No. WO 2003/003395    Patent Document 2: Japanese Laid-Open Patent Publication No. 2005-093777    Patent Document 3: Japanese Laid-Open Patent Publication No. 2000-195555    Patent Document 4: Japanese Laid-Open Patent Publication No. 2001-148242    Patent Document 5: Japanese Laid-Open Patent Publication No. 2004-111374    Patent Document 6: Japanese Laid-Open Patent Publication No. 2004-342605    Patent Document 7: Japanese Laid-Open Patent Publication No. 2004-193004